This invention relates to a method and apparatus for measuring in-situ the flux rates of soluble contaminants in sediments.
One of the most costly environmental management challenges facing the United States and other modern countries is the monitoring and management of sediments that have been contaminated through industrial activity. Contaminants of concern include both organic pollutants and heavy metals that have been released to sediments and strata underlying sediments of various water bodies including rivers, streams, wetlands, estuaries, lakes, etc. (Linz, D. G. et al., Environmentally Acceptable Endpoints in Soil, American Academy of Environmental Engineers, pp. 35-40., 1997). Management options include long term monitoring, in-situ (in-place) remediation using treatment technologies, removal and disposal, and installation of a “cap” consisting of a stratum or combination of strata (such as bentonite or bauxite) that offer passive or active resistance to the transport of contaminants to the overlying water column.
One of the most difficult problems is the empirical measurement of contaminant flux rates occurring in the actual environment. Current methods of estimating flux rates involve the removal of sediment material from the environment and the placement of this material into artificial test systems usually consisting of microcosms or columns operated in the laboratory Committee on Bioavailability of Contaminants in Soils and Sediments, National Research Council, “Bioavailability of Contaminants in Soils and Sediments: Processes, Tools and Applications”, National Academies Press, Washington, D.C., pp. 216-231 (2003); Linz, D. G. et al., Environmentally Acceptable Endpoints in Soil, American Academy of Environmental Engineers, pp. 35-40, 1997). The disadvantage of this approach is that the structure of the strata and the physical distribution of contaminants can be disrupted, making the laboratory apparatus non-representative of the actual transport regime of the existing sediment/capping system.
One method currently employed for measuring the flux rates of contaminants in sediments consists of the collection of grab samples using conventional equipment (see USEPA, “Sampling for Contaminants in Sediments and Sediment Pore Water”, Measurement and Monitoring Technologies for the 21st Century, Technology Innovation Program., http://clu-in.org/programs/21m2/sediment, pp. 1-19, (2004)) and placing the sediment in laboratory microcosms (i.e. bench scale columns or aquaria) where volumes of water can be passed through the sediment material to measure contaminant mobility. Large volumes of water are sometimes collected from these laboratory units if contaminants are of low solubility. These large volumes of water are extracted into small volumes of extractant (e.g. solvent) for purposes of concentrating contaminants to measurable levels. However, these laboratory units do not simulate the actual aquatic environment, as a result of which sediments that are collected and charged into laboratory units are disturbed and do not represent contaminant transport characteristics of the undisturbed in-situ sediment environment. In closed laboratory units, the bulk water above the sediments contains levels of contaminants approaching equilibrium with the sediment solids. In the normal aquatic environment, the bulk water provides a high dilution of the contaminant released from the sediment. Thus, the driving gradients for transport of contaminants in the natural environment are higher than observed in laboratory units unless the laboratory units are continually flushed with fresh water. In addition, laboratory microcosms that are continually flushed with water are expensive to operate and analytical data from these systems is difficult to translate into an estimate of cumulative flux through the surface of the sediment over time to calculate flux rates.